99Tc arising from spent fuel reprocessing is a major radiation concern owing to a combination of high thermal fission yield (6%), long half life (2.13×105 y), high environmental mobility in oxidized pertechnate form combined with radioactivity as a β-emitter. Further, 99Tc presents a challenge to conventional high temperature vitrification in a borosilicate glass matrix owing to its volatility at glass synthesis temperatures.
One of the methods to capture 99Tc, is to immobilize it in a suitable matrix like an iron based spinel material such as magnetite (Fe3O4) or a common corrosion product of iron and steel in aqueous or marine environments such as Goethite (FeOOH). This subject has been investigated using theoretical and experimental means. The salient findings are as follows:                1. Three Tc (IV) can replace four Fe (III) in α-Goethite (FeOOH), while creating one Fe (III) vacancy or replacement of Fe (III) by Fe (II).        2. Fe (II) is essential for reduction of pertechnate ionic species        3. Goethite conversion to magnetite is impeded by the presence of phosphate species in the waste        4. Leach rates of Tc (IV) incorporated into either magnetite/goethite structure are very low        5. Probability of Tc (IV) re-oxidation into Tc (VII) is quite small once Tc (IV) is part of magnetite/goethite crystal structure under heating in air        6. Experiments on 99Tc removal using α-Goethite (FeOOH) have been reported on lab scale. However, in these processes, ferrihydrite (Precursor to Goethite), is synthesized ex-situ under anoxic conditions and then added to the liquid waste, followed by dosing with Fe (II), usually FeCl2         7. The literature presented and also references therein contain many examples of the use of ex-situ synthesized ferrihydrite used for Tc removal, such as from Tc contaminated soils.        8. The incorporation of Tc into Fe-oxyhydroxides/oxides is well known in the literature, including the references provided and references therein. However, there is no procedure in the literature that allows a simple single step formation of these iron oxides/oxyhydroxides, without the prior ex-situ synthesis of ferrihydrite phase under anoxic conditions.        9. Tc removal using FeS route is also well reported in the literature. In this method, Tc is sequestrated in a sulphur bearing phase such as Mackinawite. However, such a sulphide bearing waste cannot be vitrified in conventional borosilicate wasteforms, which significantly limit the utility of this technique.        10. Elemental iron has also been used for reduction of Tc7+ to the less mobile Tc4+ form, using nano-iron supported on a variety of high area substrates. In these cases, it has been established using EXAFS that Tc is sorbed on the surface of the isolated nano iron particles as TcO6 octahedra, but they are not taken up into a mineral phase.        
Processes extent in the literature discusses the removal and sequestration of 99Tc by co-precipitation of iron oxides and iron oxy-hydroxides such as magnetite or goethite, however, the synthesis of ferrihydrite, a precursor to goethite, is carried out ex-situ. The reaction of FeCl2.4H2O with a pH increasing to near 7.5 using NaOH solution (1 M) yields ferrihydrite, which is unstable in air and is therefore synthesized and also stored in anoxic conditions. Such a storage protocol also implies that any contact of ferrihydrite with oxygen will result in the formation of Fe(OH)3 and associated products, including magnetite. Ex-situ formed crystalline material such as magnetite shows poor Tc uptake and this makes storage and indeed ex-situ preparation a highly involved process, which is an impediment for scale up to plant scale operations.
The other process of FeS assisted precipitation of Tc suffers from the end product being a sulphide, which is then incompatible with borosilicate glass matrices. Consequently, the sulphide wastes are disposed in cement, a waste form having a significantly shorter life than the half life of 99Tc (2.13×105 y). Additionally, there is the attendant risk of Tc remobilization by oxidation from sulphide wasteforms.
The third method extant in the literature is the use of zero valent iron for reductive sequestration of Tc. Indeed, this method has been used for reductive removal of various metals including Cr, Pb and As to name a few from various liquid streams. In reference 11 listed in prior art, it has been proven using Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) that Tc is reduced from Tc (VII) to Tc(IV) and also Tc (V) by elemental nano iron. Isolated TcO6 octahedra are then adsorbed on the surface of iron nano-particles. In such a state, they are not part of a mineral lattice. Therefore, potential reoxidation and remobilization risk remains viable in such an immobilization strategy.
The proposed process avoids the ex-situ ferrihydrite synthesis steps and the associated anoxic conditions. Instead, goethite/magnetite is generated in-situ by the corrosion of mild steel wool introduced into the intermediate level waste (ILW) with ILW volume to mass of steel ratio (V/m) ranging from 100 ml·g−1 to 1000 ml·g−1 with pH between 2-8.
Of note is the fact that the sequestered Tc will be accommodated in mineral phases, which may reduce remobilization risks. After a holding time of ˜4 h-48 h, more than 99% of the 99Tc in the ILW is taken up either by goethite or magnetite phases formed as corrosion products. These corrosion products can be directly disposed by vitrification into durable borosilicate wasteforms, since the 99Tc is fixed in the crystal lattice of the corrosion product and therefore, cannot volatilize or re-oxidize as easily. As an additional benefit, waste volume generation is small. Further, since the quantity of mild steel lost in corrosion is quite small, the mild steel wool can be continuously re-used for in-situ generation of goethite/magnetite.